1. Field of the Invention
The present invention relates to an optical displacement sensor and an external force detecting device, and particularly to an optical displacement sensor which detects relative displacement between a reference object and a measurement object based on displacement of a light reception position, and further to an external force detecting device which detects an external force applied to the measurement object based on a signal outputted from the optical displacement sensor.
2. Description of the Related Art
An external force detecting device, such as a six-axis optical force sensor, is conventionally known, in which a displacement amount of an action section to receive an external force, namely a measurement object, relative to a support section to support the action section, namely a reference object, is detected by an optical displacement sensor, and an external force applied to the action section is measured according to an output signal from the optical displacement sensor.
For example, a six-axis optical force sensor comprises optical displacement sensors to measure a six-axis direction displacement, based on which a six-axis force is calculated. Specifically, such a six-axis optical force sensor comprises three optical displacement sensors, each of which uses an optical sensor unit and is capable of measuring a two-axis (X and Y) direction displacement, thereby enabling measurement of a six-axis direction displacement. The optical sensor unit comprises a light emitting diode (LED) as a light source and a photodiode (PD) assembly as a light receiving element, such that the LED opposes the PD assembly with their respective optical central axes aligned to each other. The PD assembly is composed of four PD's and receives light emitted from the LED at its center area equally shared by the four PD's, whereby displacement of light receiving position at the PD assembly, that is to say relative positional displacement between a component attached to the LED and a component attached to the PD assembly can be detected in the optical displacement sensor. In the six-axis optical force sensor, a six-axis force applied between the component attached to the LED and the component attached to the PD assembly is measured according to an output signal from each of the optical displacement sensors.
FIG. 1 is a plan view of a conventional six-axis force sensor as disclosed in Japanese Patent Application Laid-Open No. H03-245028. A six-axis force sensor 101 shown in FIG. 1 is structurally composed of a cylindrical main body, and top and bottom lids (not shown). The main body is constituted basically by a frame 105, which integrally includes: a cylindrical support section 102; an action section 103 located centrally inside the support section 102 and adapted to receive an external force; and three elastic spoke sections 104 crookedly structured so as to be duly deformed elastically in all directions and supportably connecting the action section 103 to the support section 102. The frame 105 is made of a single piece of an aluminum alloy material and shaped by cutting and electric discharge machining. The support section 102 and the action section 103 are fixedly attached respectively to two components to which a measurement force is applied, and when a force applied acts on the six-axis force sensor 101 structured as described above, a micro-displacement with respect to three-axis direction and a micro-rotation with respect to three-axis rotational direction are generated between the support section 102 and the action section 103.
The six-axis force sensor 101 further includes three light sources 106 disposed at the inner circumference of the support section 102 at 120 degree intervals (i.e. at an equi-angular distance), and three optical sensors (light receiving elements) 108 disposed at the action section 103 at 120 degree intervals (i.e. at an equi-angular distance) so as to oppose respective three light sources 106 with mutual optical axes aligned to each other. Each optical sensor 108 and each light source 106 disposed opposite to the optical sensor 108 make up an optical sensor unit (optical displacement sensor) 109.
FIG. 2 is an explanatory perspective view of the optical sensor unit (optical displacement sensor) 109 of FIG. 1. As shown in FIG. 2, each of the optical sensors 108 is constituted by a PD assembly composed of four PD's 108a. The light sources 106 disposed so as to oppose respective optical sensors 108 are each constituted by an infrared high-intensity LED with a pinhole aperture provided at its front face, and light emitted from the LED 106 and passing through the pinhole aperture propagates diffusedly and impinges on the center portion of the optical sensor 108 so as to be substantially equally irradiated on all the four PD's 108a. If the support section 102 and the action section 103 are displaced relative to each other by an external force, then the light emitted from the LED 106 is irradiated unequally on the four PD's 108a, and light amounts received at respective four PD's 108a are measured for calculation of relative displacements with respect to X- and Y-axis directions. And, the six-axis force sensor 101 calculates forces with respect to six-axis directions according to the above-calculated relative displacements, and a signal is outputted therefrom.
However, the aforementioned conventional optical displacement sensor, and the aforementioned six-axis force sensor, i.e., external force detecting device, incorporating the conventional optical displacement sensor has the following problems when respective optical axes of the LED 106 as the light source, and PD assembly 108 to receive light emitted from the LED 106 are aligned to each other. Specifically, in the alignment work, while positional adjustment with respect to the X- and Y-axis directions is easy, rotational adjustment about a Z-axis perpendicular to the X- and Y-axes (to precisely bring the cross-shaped boundary formed by the four PD's 108a in line with the X- and Y-axes) is very difficult
Conventionally, the rotational adjustment has to be carried out such that an LED and a PD assembly are tentatively arranged, and misalignment in the rotational direction about the Z-axis is checked and corrected based on a signal from the PD assembly, which is gained by causing an action section at which either the LED or the PD assembly is disposed to be displaced in the X- and Y-axis directions. This involves a lot of works, and requires an immense amount of time and effort, especially when displacement amount is large.